"This is particularly important for smaller touchscreens, where screen real estate is limited. If we can remove mode buttons from the screen, we can make room for more content or can make the remaining buttons larger."

TapSense works by classifying the sounds the different parts of the finger make as they strike the screen. All it needs is an additional, inexpensive microphone - those already used in phones are optimized for capturing voices.

Other applications, says the team, could include a painting app that uses different tapping modes to control colors, or switch between drawing and erasing without having to press buttons.

The technology also can use sound to discriminate between tools made from diffferent materials, such as as wood, acrylic or polystyrene foam.

This would enable people using styluses made from different materials to collaboratively sketch or take notes on the same surface, with each person's contributions appearing in a different color.